Very large antenna array systems, such as multiple input multiple output, MIMO, systems, provide the opportunity for spatial division multiple access, SDMA, in which each individual user may be served with the same time-frequency resource as other users. This multiple access technique requires accurate and coherent control of both amplitude and phase over the antenna array, which is an issue often solved by the use of high speed, high resolution digital to analog converters, DACs.
FIG. 2 shows an antenna arrangement 200 using a fully digital beam-forming scheme, which requires one high resolution DAC per antenna-branch, shown in FIG. 2 as “High Res DAC”. The respective output signals from the DACs are processed by mixers fed by a signal from an oscillator, OSC, and amplified by power amplifiers, PA, before being transmitted via antenna elements. FIG. 2 illustrates the case of a 4-dimensional antenna array, i.e., an antenna array with four antenna elements, but the concept is readily extendable to an arbitrary number N>1 of antenna elements.
High speed, high resolution DACs tend to consume a significant amount of power. For some example DAC circuits, every added bit, i.e., increase in DAC resolution, doubles the chip-area and the power consumption of the DAC. State-of-the-art high speed and high resolution DACs capable of running at high bandwidths usually consume power in the range of 1-2 Watts, which, when multiplied by the number of antenna elements in a large antenna arrangement, can result in a power consumption on the order of several hundreds of Watt's.
A power-conserving alternative to the fully digital beam-forming scheme discussed in connection to FIG. 2 is analogue beam-forming. FIG. 3 shows an antenna arrangement 300 with an analog beam-forming mechanism using phase-shifters, F, which in turn are controlled by a pre-coder that outputs control voltages c1, c2, c3, c4.
Analog beam-formers, such as the one 300 illustrated in FIG. 3, often comprise either passive structures such as a Butler matrices which constrains the spatial duplexing into a number of fixed beams, or analogue phase-shifters, which requires advanced calibration, since these usually comprise nonlinear analogue components. Further, analog phase-shifters do not provide means for control of the amplitude of the respective output signals. In order to provide beam-forming with amplitude control of the different antenna signals, either the PAs need to be able to provide fine-grained amplitude control, or an additional set of amplifiers are needed in the design. Both of which options add to the complexity of the antenna arrangement.
Consequently, some present antenna arrangements configured for digital beam-forming of a transmit signal either consume significant amounts of power, or are constrained in their spatial duplexing ability.